The joint FBI and Homeland Security intelligence warning, issued Monday, lists indicators that could tip off police to homemade hydrogen peroxide-based explosives, such as people with burn marks on their hands, face or arms; foul odors coming from a room or building; and large industrial fans or multiple window fans. The warning, obtained by The Associated Press, also said that these homemade explosive materials can be hidden in backpacks, suitcases or plastic containers.